Today printed wiring boards and circuit boards have smaller wettable surfaces to be solder coated and joined. In co-pending application Ser. Nos. 07/961,781 and 07/860,316 is disclosed gas shrouded wave soldering wherein a solder wave has a cover with a slot for the solder wave to project therethrough and wherein a shield gas, preferably nitrogen and substantially oxygen free, is supplied underneath the cover so that the solder is applied to circuit boards and the like when they are substantially blanketed by the shield gas. In the aforesaid co-pending applications, soldering occurs in a shield gas atmosphere, referred to as an inert gas and this includes nitrogen. The shield gas prevents oxides forming on the liquid solder surfaces, and in another embodiment, a reducing gas such as hydrogen is included in the shield gas in a non-explosive mixture, to provide fluxing to the surfaces to be soldered or solder coated. U.S. Pat. No. 5,044,542 discloses shield gas wave soldering wherein shield gas or a reducing gas blankets the solder wave during the soldering step. The benefits of soldering in an atmosphere which is substantially oxygen free are known.
In our co-pending applications a cover is provided to at least partially cover a solder reservoir and shield gas passing on both sides of the solder wave blankets the solder wave and the solder within the reservoir. By utilizing a cover one is able to reduce the flow of shield gas for blanketing the solder wave and the circuit board or wiring board passing therethrough compared with a tunnel or enclosure. Thus, wettable surfaces to be solder coated such as circuit tracks, pads, component leads, surface mounted components and metallized holes in boards with pins therein are kept substantially free of air. The shield gas which is preferably nitrogen and is substantially oxygen free, is introduced below the cover and passes up through a slot through which the solder wave projects on both sides of the solder wave to blanket the solder wave and also blanket the bottom surface of a circuit board passing through the solder wave as well as the edges of the circuit board.
Circuit boards are conveyed to pass through a solder wave by different types of conveyors. In one embodiment the circuit boards are supported by gripping fingers from two separate parallel conveyors. In another embodiment the circuit boards are supported on pallets, fixtures or frames which in turn are supported either by two separate conveyors gripping side edges, or by a single conveyor. If two parallel conveyors are used with gripping fingers, then there is provision to adjust the distance between the conveyors to take into account variable widths of conveyors or different sizes of pallets, fixtures or frames to support circuit boards.